The present invention generally relates to an electric motor and more particularly, to a brushless motor to be employed, for example, as a source for driving a fan to cool an electronic appliance or the like.
With the development of electronic appliances, fans to be employed for cooling thereof play an important part in order to maintain such electronic appliances in a highly reliable state. More specifically, it is normally required that such a fan should have a longer life, and a higher dependability than the electronic appliance system applied therewith. Accordingly, an induction type motor has been conventionally employed for this purpose in many cases. However, in recent years, a dc brushless motor has become widely employed in replacing the induction type motor as referred to above, owing to such advantages thereof that it can be formed in a compact size through standardization and high speed operations without being influenced by the power source voltages, frequencies, etc., while the amount of cooling air therefrom may be readily adjusted according to temperatures of the electronic appliances.
Meanwhile, the fan as described above is required to be inexpensive, since it is incorporated into the electronic appliance merely as an additional part. Therefore, in the conventional brushless motors of this this type, it has been an important technical point to construct the motor in a compact form with the smallest number of parts, and the lowest cost as possible.
For such a motor, since a small starting torque is sufficient for dealing with a fan load, a brushless motor of a 2-phase half wave driving system or a single-phase full wave driving system, or that of a 4-phase half wave driving system or a 2-phase full wave driving system which is devised to remove a "dead point" of torque caused by the rotor position during starting has generally been employed. In connection with the above, one example of a brushless motor of the 2-phase half wave driving system is disclosed at pages 794 to 800 of National Technical Report, Vol. 26, No. 5 (October, 1980).
Particularly, simplification of construction of a motor driving circuit is essential to a fan driving motor for reduction of manufacturing cost and achieving a compact size. The simplest construction for the driving system of a brushless motor is that of the single-phase half wave driving system. In this system, however, since there is a zone at which no torque is produced depending on the position of the rotor, it is necessary to start the rotation of the shaft by a mechanical means. Once the motor has started to rotate, it continues under a small load, owed to the rotational inertia of the rotor by overcoming the zone, not by generating torque. The above described motor type is suitable for application to a clock or the like, because the clock deals with a small load, without repeated starting and stopping.
Subsequently, as one example of conventional constructions, a brushless motor of a single-phase half wave driving system will be described with reference to FIG. 7. FIG. 7 shows structures of a driving circuit and a motor portion thereof in an orthogonal cross section, with its axial direction being aligned with the direction perpendicular to the paper surface of the drawing. Although the motor in the drawing is represented by two poles, a similar function is also available in multi-poles.
In FIG. 7, the known brushless motor includes a stator pole 1, an exciting coil 2 wound around the stator pole 1 with one end of the exciting coil 2 being connected to the .sym. side of a dc power source 7 through a switching element 6 and the other end is connected to the .crclbar. side of said dc power source 7, and a rotor 3 of a permanent magnet is fixed on a shaft 4 for rotation in a direction indicated by an arrow. In the drawing, polarities N and S represent the relation between the polarities to be produced by an exciting current that is caused to flow through the exciting coil 2 of the stator pole 1 at a certain instant, and the polarities of the rotor magnet. For a rotor position detector 5, a hall element or magnetic reluctance element is normally employed in many cases. Now, on the assumption that the rotor position detector 5 is to drive a signal responsive to the N polarity of the rotor magnet 3, a moment is represented in the drawing in which, with respect to the rotational direction as shown, a switch ON signal is fed to the switching element 6 so as to pass the current through the exciting coil 2. In the state as shown in FIG. 7, the poles of the stator pole 1 and the rotor magnet 3 confront each other at the same polarities, with no torque being produced. At this moment, on the assumption that time t is equal to zero, when the rotor magnet 3 is rotated in the direction indicated by the arrow with a lapse of time, the torque reaches a maximum valve when the rotational angle is between zero and (.pi./2) before being reduced to zero at .pi.. Since the rotor position detector 5 is to obtain a signal from the S pole region of the rotor magnet 3 at this time, the switching element 6 is brought into OFF state, and no exciting current will flow. Accordingly, torque is not generated by the rotor magnet 3 between a rotational angle from .pi. to 2.pi.. Such a state is shown in a graphical form in FIG. 8, in which +T represents the torque in a regular direction, while -T denotes the torque in the opposite direction.
Although the brushless motor of the single-phase half wave driving system is simple in construction, it has a disadvantage in that there is a rotor position where no torque is produced as described above.